1. Field of the invention
The present invention relates, in general, to an insulation technology for polysilicon film used as a charge storage electrode of trench capacitor in a semiconductor device, more particularly to improvements in device reliability along with the technology.
2. Description of the Prior Art
Hereinafter, description will be given for a conventional method for insulating a polysilicon film serving as a charge storage electrode of trench capacitor from an upper layer thereof with reference to FIGS. 1A through 1B, wherein well known processing steps have not shown such as the formation of active regions and field stop ion-implantation region, etc. needed for the formation of trench capacitor of DRAM, for convenience.
Firstly, over a semiconductor substrate 1 covered with an oxide film 2 is formed a field oxide film 3 by local oxidation of silicon (hereinafter, "LOCOS") process and then, a nitride film 4 is formed over the resulting structure including a bird's beak formed by the LOCOS, as shown in FIG. 1A.
Next, an etch process is applied to the nitride film 4, the oxide film 3, the field oxide film 2 and the semiconductor substrate 1 so as to form a trench at a predetermined portion including the bird's beak, and a thin oxide film 5 is formed on the surface of the semiconductor 1 exposed during the formation of the trench, followed by the filling of the trench with polysilicon film 6, as shown in FIG. 1B.
Subsequently, the surface of the polysilicon film 6 is oxidized to form an oxide film 7 which is utilized as a dielectric film, the remaining nitride film serving as a mask, as shown i n FIG. 1C.
Finally, the nitride film is removed and then gate electrodes are formed on predetermined portions of the resulting structure, as shown in FIG. 1D.
By the portrayed conventional method, which comprise forming a trench at a predetermined portion including a bird's beak caused during the performance of LOCOS for the formation of field oxide film, burying a polysilicon film in the trench, and forming an oxide film on the upper portion of the polysilicon film to insulate a charge storage electrode, the insulation for the polysilicon film as a charge storage electrode cannot be secured satisfactorily. For example, there is generated a phenomenon such that the peripheral thickness of the oxide film formed on the polysilicon film filling the trench at peripheral portions may be thinner than its central thickness due to the existence of the nitride film around the trench, resulting in affecting adversely the insulation of the device (referring to "A" portions in FIG. 1C).
This failure of insulation for the polysilicon film used as a charge storage electrode in a semiconductor device results in the degradation of device reliability.